1. Field of the Invention
The present invention relates to a gas sensor element, and more particularly to a gas sensor element suitably used for a gas sensor to measure oxides such as NO, NO2, SO2, CO2, and H2O and combustible gases such as CO and CnHm, contained in automobile exhaust emissions and the atmosphere. The invention also relates to a nitrogen oxide (NOx) sensor utilizing such a gas sensor element.
2. Discussion of Related Art
There have been proposed various measuring methods and devices for detecting concentrations of desired gas components such as those described above contained in a measurement gas. For instance, JP-A-8-271476 (Patent Document 1), JP-A-11-237362 (Patent Document 2), and JP-A-2000-28576 (Patent Document 3) disclose a measuring device configured to measure an amount of a desired measurement gas component contained in the measurement gas by measuring an amount of oxygen generated by reduction or decomposition of the measurement gas component. More specifically described, the disclosed measuring device uses a gas sensor element to detect the concentration of the desired gas component contained in the measurement gas. The gas sensor element has a laminar structure formed by laminating a plurality of solid electrolyte layers each having a suitable thickness integrally on each other. Within the laminar-structured element, there are provided a first internal space into which the measurement gas is introduced through a first diffusion controlling passage, a second internal space into which an atmosphere in the first internal space is introduced through a second diffusion controlling passage and in which the measurement gas component existing in the atmosphere is reduced or decomposed, first oxygen pumping means to control an oxygen partial pressure in the first internal space, second oxygen pumping means to pump the oxygen out of the second internal space, and current detecting means to detect a pumping current flowing upon a pumping action of the second oxygen pumping means. The gas sensor element detects the amount of the desired measurement gas component on the basis of the pumping current detected by the current detecting means.
The gas sensor element disclosed in the above-indicated Patent Publications includes at least one pair of electrodes arranged to measure an electromotive force based on a difference in an oxygen concentration between atmospheres, for the purpose of detecting the oxygen concentration in the atmosphere, controlling the pumping action performed by the oxygen pumping means disposed in the element. The at least one pair of electrodes are provided in the solid electrolyte layers which are laminated integrally on each other, and cooperate with the above-indicated internal spaces, diffusion controlling passages, oxygen pumping means, etc., to constitute a sensor layer.
In the gas sensor element having the sensor layer described above, in general, a suitable heater layer is laminated integrally on the sensor layer to provide an integral element structure. The heater layer is constituted by including: a heater element having a heat generating portion; and an electrically insulating layer disposed so as to enclose the heater element. By supplying an electric current to the heater element, at least a portion of the sensor layer at which the pair of electrodes for measuring the electromotive force are provided is heated to a prescribed temperature, whereby an operation of detecting the amount of the desired measurement gas component can be effectively conducted.
The above-described laminar-structured gas sensor element which has been conventionally proposed suffers from the following problem when used for measuring the measurement gas such as exhaust emissions. That is, when an electric current is supplied to the heater element of the heater layer of the gas sensor element and a prescribed pulse voltage is applied thereto, noise signal is generated, so that an output value of the electromotive force detected by the pair of electrodes provided in the sensor layer is influenced by the noise signal and undesirably suffers from fluctuations. In particular, while there is a tendency of reduction in a frequency of the voltage applied to the heater element in accordance with recent digitalization of a control circuit for the gas sensor element, such reduction in the frequency of the voltage to be applied to the heater element causes the fluctuations in the output value described above to become large. In this instance, where the desired measurement gas component in the measurement gas is detected based on the fluctuated output values by controlling the oxygen pumping means, for instance, the gas sensor element does not ensure a sufficiently high degree of accuracy of detecting the measurement gas component.    Patent Document 1: JP-A-8-271476    Patent Document 2: JP-A-11-237362    Patent Document 3: JP-A-2000-28576